1. Field of the Invention
The present invention relates to a polymer scale preventive agent, and a polymerization vessel and method for preventing polymer scale deposition for use in polymerization of vinyl chloride or a monomer mixture containing vinyl chloride as a major component.
2. Description of the Prior Art
In processes of preparing polymers by polymerizing a monomer in a polymerization vessel, the problem that polymer deposits on the inner wall surface and so forth in the form of scale, is known. The deposition of the polymer scale on the inner wall results in disadvantages that the yield of the polymer and the cooling capacity of the polymerization vessel are lowered; that the polymer scale may peel and mix into manufactured polymer, thereby impairing the quality thereof; and that removal of such polymer scale is laborious and hence time-consuming. Further, since the polymer scale contains unreacted monomer and operators may be exposed thereto, which may cause physical disorders in the operators.
Heretofore, to prevent polymer scale deposition on the inner wall surface and so forth of a polymerization vessel, methods in which suitable substances are coated on the inner wall surface, etc. as a scale preventive agent, are known. The scale preventive agents include, for example, a dye or pigment (Japanese Patent Publication (KOKOKU) Nos. 45-30835(1970).
In the meantime, recently, in regard to formed or molded products of vinyl chloride polymers, in particular clear sheets and bottles, improvement in quality with respect to properties such as color, fish eye, bulk specific gravity, gelation rate, thermal stability, etc. is increasingly required by the manufacturers thereof. It is therefore expected to develop a scale preventive agent which does not affect adversely the properties of product polymers.
In the case said dye or pigment is used as a scale preventive agent, however, properties of product polymers are adversely affected as exemplified by (i) and (ii) below.
(i) The coating composed of the scale preventive agent may be readily dissolved, and therefore produced polymers may be colored or may have a lowered bulk specific gravity. Coloration of a polymer is desired to be low to such an extent that the lightness L in the Hunter's equation described in JIS Z 8730 (1980) is at least 70. PA1 (ii) Since adhesion of the coating to the inner wall surfaces of a polymerization vessel is weak, the coating may be partly peeled and lost and polymer scale may deposit on the areas where the carting has been lost, causing increase in fish eyes when the product polymer is processed into formed products such as sheets, etc. PA1 (A) a naphthoquinone natural dye and PA1 (B) a polyvinyl alcohol with a saponification degree of at least 50 mol %.